Liquid crystal displays (LCD) typically have two substrates, one or both of them being transparent, between which is sandwiched a liquid crystalline material that blocks or transmits light. The liquid crystalline material is generally twisted nematic. Twisted nematic displays pass or reflect incident light, while cholesteric displays usually reflect light.
The liquid crystalline material responds to applied electric fields. Cholesteric materials switch between a reflective state referred to as the planar cholesteric and a transparent state referred to as the focal conic state. When the molecules align in the planar cholesteric state in response to an electric field, they reflect light of a particular wavelength. The wavelength of the reflected light is proportional to the pitch distance of the material.
Generally, a cell with a given material will have a pitch distance corresponding to red, green or blue. Conventional approaches provide separate elements of each color. The separate elements may be stacked upon one another to generate the full color reflected light output. Alternatively, the three elements may be located next to each other spatially, with the combination of their colors being manipulated by optics, including color filters. Either of these approaches reduces the brightness of the display.
Even with these limitations, cholesteric displays have advantages over other types of LCDs, especially for mobile applications. Being reflective, they do not require power hungry back lights. Bistable cholesteric displays will remain in whichever state they are placed, without refresh, and even upon removal of power. This further reduces the need for power.